Lipohypertrophy is a thickened, enlarged or raised portion of skin's fatty underlayer where insulin injections are given. It's a kind of lipodystrophy. Continuing to inject into the same area can be one cause of this; the need to alter injection techniques can be another.
The person shown in this photo link below has had diabetes since age 17. Throughout the many years she used insulin, she injected it into only the two areas which are overgrown with fatty (adipose) tissue. Insulin itself stimulates fat storage. The photo illustrates what continuing to inject it in the same places over an extended time period can do.
Changing or rotating the site one injects insulin is a good way to prevent lipohypertrophy and lipoatrophy, as well as preventing possible absorption problems. Insulin does not absorb well from these thickened areas.