Why does my car shakes like it wants to stall when foot is on break pedal at a stop light?

I have a 2000 Nissan Altima Gxe Codes stored are for knock sensor and catalytic converter, occasionally when I come to a stop at a stop light and my foot is on the break my car shakes like if it wants to stall If I give it slight gas from the excelerator it is fine and stops shaking. I know I need the knock sensor and catalytic converter fixed but my mechanic told me they are just emission codes no need to rush getting it fixed, I am wondering if this shaking has anything to do with either of those issues? This has happened to me 3 times in 6 months I drive at least 100-150 miles a month at most. P.s I am a female and Have no car knowledge so Please explain any mechanic terminology I might not understand