They planned to visit their grandparents this weekend.
Here 'to visit' is a/an -

• They planned to visit their grandparents this weekend.
- Here 'to visit' is an Infinitive.

• Infinitive: 
•  Infinitive হচ্ছে verb এর base form অথবা to + base form.

• Infintive দুই রকম হতে পারে-
১. To-যুক্ত infinitve এবং
২. To-বিহীন infinitive বা Bare Infinitive.

More Example:
1. She wants to learn how to play the guitar.
- Here, "to + learn" is an Infinitive.

2. She hoped to find a solution to the problem.
- Here, "to + find" is an Infinitive.