5.21 understand that condensation polymerisation produces a small molecule, such as water, as well as the polymer

Two monomers come together by loosing a molecule. Atoms from each monomer join together to make the molecule: commonly a H atom from one and a OH molecule from another form water. The two monomers then join together, making a polymer.

5.20 understand that some polymers, such as nylon, form by a different process called condensation polymerisation

Two monomers join together when they loose a small molecule made up of atoms from both monomers.
Commonly, an H atom from one and and OH molecule from the other form water (hence condensation reaction) and the two monomers become joined.

askmichelle

This is what happens to make nylon:

chemguide

5.19 explain that addition polymers are hard to dispose of as their inertness means that they do not easily biodegrade

Polymers are saturated so they don't react. This means they don't decompose easily.

5.18 describe some uses for polymers, including poly(ethene), poly(propene) and poly(chloroethene)

polyethene: plastic carrier bags; plastic bottels
polypropene: crates; ropes
polychlroethene: piping; cable insulation.

5.17 deduce the structure of a monomer from the repeat unit of an addition polymer

A monomer that is repeated in a polymer looks much like the repeat unit; apart from, instead of having an empty bond either end, it has a double bond in the middle.

askmichelle

5.16 draw the repeat unit of addition polymers, including poly(ethene), poly(propene) and poly(chloroethene)

The repeat unit is the structure that is repeated to form a polymer.
(ignore PTFE on this diagram)

GCSEscience

5.15 understand that an addition polymer is formed by joining up many small molecules called monomers

monomers are alkenes with a double bond. If this bond is broken there can be other things bonded, if a carbon from another monomer is bonded in then you can create a chain; do this many times can you have a polymer.