Why do I hear water in my bathtub when toilets are flushed? Is this a major problem?

Is it a gurgle, likely a partially clogged vent stack. Major problem, no not now but later yes.

Why do I hear water in my bathtub when toilets are flushed? Is this a major problem?

Bob's bang on! The gurgle you hear is the toilet attempting to bent through the tub trap. The solution would be to snake the lavatory vent from the roof.
Rent a sewer machine,(see image) and send the snake down the vent until it reaches the base and 16 more feet. Good luck. Tom