They are somewhat like tools. Imagine trying to losen a screw without a screwdriver. It might be possible, but much more effective with the tool. For example enzymatic catalysis: a molecule that gets trapped in the enzyme might be “stretched” by electrostatic forces, which makes it easier for other molecules to interact with it. The stretch-mechanism might be so strong, that it even tears the molecule appart. Similar reactions occur in activated carbon for example.
Other kinds of catalysts can help reactions by making it easier to react by transforming one of the agents first, e.g. acids/bases that modify protons for easier access.
There are many ways to assist in a reaction, the gist of it is lowering the energy needed to get something done.
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