Wedding crashing is the act of attending a wedding celebration without an invitation, particularly when the persons who turn up have a profound impact.

Hello! It means that you attended a wedding without being invited to it. It's also formally known as "Gatecrashing".

It means that you showed up (to a wedding or party) uninvited (without being asked).

I arrived uninvited.

To crash a wedding means to show up uninvited, as well as to make things from uncomfortable to downright awful for the bride and groom.