Basic knowledge of the aerodynamic principles is highly recommended before getting involved in building and/or flying model aircraft.

The air density depends on the atmospheric pressure and on the air temperature. For example, if a glider glides 25 meters from a given altitude during low atmospheric pressure, it may glide 40 meters during high pressure.

A flying aircraft is subject to a pressure depending on the airspeed and the
air density.
This pressure increases exponentially with increasing of the airspeed.
The aircraft's resistance to the airflow (drag) depends on the shape of the
fuselage and flying surfaces.
An aircraft that is intended to fly fast has a thinner and different wing profile
than one that is intended to fly slower.
That's why many aircraft change their wings' profiles on landing approach
by lowering the flaps located at the wings' trailing edge and the slats at the
leading edge in order to keep enough lifting force during the much lower
landing speed.

The wings' profile of a slower aircraft is usually asymmetric, this causes the air
on the wings upper side to accelerate downwards, making the pressure on the
upper side lower than the underside, thereby a lift force is created.

The lift force of a symmetric profile, is based on the airspeed and on a positive
angle of attack to the on-coming flow.
The air always flows toward areas of lower pressure and away from areas of
higher pressure, thus the air over the wing top accelerates as it enters the lower
pressure region (where the air curves toward the wing), whereas the air under
the wing slows down as it enters the higher pressure region (where the air curves
away from the wing).
So, one may also say that the wings create lift by reacting against the air flow,
driving it downwards, producing downwash.
The top of the wing is often the major lift contributor, usually producing twice as
much lift as the bottom of the wing.

The following picture shows the airflow through two wing profiles.

The uppermost profile has a lower angle of attack than the lowest one.
When the air flows evenly through the surface is called a laminar flow.
A too high angle of attack causes turbulence on the upper surface and
dramatically increases the air resistance (drag) this may result in an
abrut loss of lift, which is known as stall.

Summarising:
The aircraft generates lift by moving through the air.
The wings have airfoil shaped profiles that create a pressure difference
between upper and lower wing surfaces, with a high pressure region
underneath and a low pressure region on top.
The lift produced will be proportional to the size of the wings, the square
of airspeed, the density of the surrounding air and the wing's angle of
attack to on-coming flow before reaching the stall angle.

How does a glider generate the velocity needed for flight?
The simple answer is that a glider trades altitude for velocity.
It trades the potential energy difference from a higher altitude to a lower
altitude to produce kinetic energy, which means velocity.
Gliders are always descending relative to the air in which they are flying.

How do gliders stay aloft for hours if they constantly descend?
The gliders are designed to descend very slowly.
If the pilot can locate a pocket of air that is rising faster than the
glider is descending, the glider can actually gain altitude, increasing
its potential energy.

Pockets of rising air are called updrafts.
Updrafts are found when the wind blowing at a hill or mountain rises to
climb over it. (However, there may be a downdraft on the other side!)
Updrafts can also be found over dark land masses that absorb more
heat from the sun than light land masses.
The heat from the ground heats the surrounding air, which causes the
air to rise. The rising pockets of hot air are called thermals.

Large gliding birds, such as owls and hawks, are often seen circling
inside a thermal to gain altitude without flapping their wings.
Gliders can do exactly the same thing.