The Institution of Mechanical Engineers is a big group of really smart people who work together to make machines run better and safer. They do this by studying and learning about how machines work and how to make them work better. They also make sure that machines are safe to use and don't hurt people.
Think of it like being part of a big club where everyone is interested in how machines work and how we can make them better. Imagine you and your friends are building a big Lego tower, but you notice that it's not very stable and keeps falling apart. The Institution of Mechanical Engineers are like the grown-up version of your friends who are really good at Legos! They'll help you figure out how to make the tower stronger and more stable so that it doesn't fall over anymore.