Meaning of “top sth up” - Learner’s Dictionary

top sth up

UK US top sth off phrasal verb with top us uk /tɒp/ verb [ T ] present participle topping, past tense and past participle topped

to add more liquid to a container in order to make it full

to add more of something, especially money, to an existing amount to create the total you need:

Can I top up my mobile phone here?

(Definition of “top sth up” from the Cambridge Learner’s Dictionary © Cambridge University Press)