Lumps in cats can be anything from inflammation and cysts to different types of tumour. Some soft tissue tumours in cats can grow infiltratively into the surrounding tissue, which means an early assessment can matter for how any operation is planned.
The vet looks at the location, size and mobility of the lump and the appearance of the skin, and may recommend a cell sample or a tissue sample. If your cat has recently had an injection or vaccination, it is also helpful to say where and when it was given. Not every lump needs surgery, but each one first needs to be assessed on its own merits.
Book an appointment if you find a new lump, or if an existing change grows, becomes harder, feels fixed, becomes sore or bothers your cat. Try to record the size at home so that changes over time are easier to judge.