Is an idiom which means: To get married
For example, my sister and her husband tied the knot a few years ago.
They are definitely not a traditional couple; they met online, she proposed to him, and their wedding invitation had a picture of two dodos on it. She’s wore a 1950’s wedding dress and they used a vegan mobile catering service for their reception. The guests were asked not to give any wedding gifts and their two sons wore cartoon character costumes.
Inglés Málaga 