the 5 inch electric fence insulators mean the insulators will keep the polywire, wire, polytape or polyrope around 5 inch away from the steel T-post,  please see the following pictures, these are the 5 inch electric fence insulators, there are 2 types based on the installation direction on the steel T-post.

We use the 5 inch electric fence insulators, because the steel T-post is conductive, the polywire, wire, polytape or polyrope will be more close to the steel T-post if we use the short electric fence insulators less than 5 inch.  the 5 inch electric fence insulator can keep the polywire, wire, polytape or polyrope won’t contact the steel T-post when the animal touch the polywire, wire, polytape or polyrope or bad weather.