Steric effects arise from the fact that each atom within a molecule occupies a certain amount of space. If atoms are brought too close together, there is an associated cost in energy due to overlapping electron clouds (Pauli or Born repulsion), and this may affect the molecule's preferred shape (conformation ) and reactivity. Steric strain (also known as Van der Walls strain) has to do with atoms being really close to one another and having "repulsion" between them. For instance, say you had a benzene ring with 2 tert-butyl groups (a -C(CH3)3 group, with one carbon being bound to 3 methyl groups). Let's also say that these groups are attached to positions 1 and 2 on the ring, or right next to each other. Because a tert-butyl group is very large, compared to, say, a methyl group or hydrogen, the electrons contained in one tert-butyl group could get really close to electrons in another tert-butyl group. When both groups get really close, they start to repel each other, because electrons repel other electrons. That's what we mean by steric strain. It's when large groups put a electronic strain on other groups due to their large size and numbers of electrons. With benzene having two tert-butyl groups next to each other, there is a lot of electronic repulsion, or steric strain, between the two tert-butyl groups.