Steric effects are the effects seen in molecules that come from the fact that atoms occupy space. When atoms are put close to each other, this costs energy. The electrons near the atoms want to stay away from each other. This can change the way molecules want to react. It can also change the shape (or conformation) of the molecule. The amount of space that a group of atoms takes is called the "steric bulk".
An example of steric effects is steric hindrance. This is when a large group in a molecule makes reactions not work. For example, an SN2 reaction does not happen on carbon atoms that have three substituents. Steric hindrance however can also be a positive thing. Sometimes, chemists would like a reaction to happen in one place rather than another one. A bulky group can block a position. It can also change the shape such that only one diastereomer from a reaction is made.