Foamy engine oil can mean that you overfilled the engine with too much oil or if it's foamy and looks white or light colored and milky it often means that you have a blow headgasket and or coolant is mixing with the oil.
Milky looking engine oil that looks white or light colored indicates a serious issue and means that coolant is getting mixed in with your engine oil and if not fixed it will quickly lead to engine failure.
If the oil is just foamy or bubbly but it's not discolored and otherwise looks normal then it's a sign that the engine has been overfilled with too much oil.
When an engine is overfilled with too much oil it causes the oil to be mixed and churned and aerated by the crankshaft which causes the foamy appearance of the oil.
If you don't drain the excess oil out to the proper engine oil level it will eventually lead to engine bearing failure and the bearings of the engine will seize up and could even cause the engine block to break or a rod to throw through the oil pan or engine block.
Foamy engine oil cannot lubricate properly so it can harm the engine.
Overfilling your engine oil slightly such as in half a quart or less is not always a problem.
But overfilling the engine oil too much is bad and can cause engine failure so you must drain any excess engine oil to prevent engine failure which is a costly repair.