Controlled practice is a stage in a lesson where learners practise new language in a limited form. It can be compared to free practice, which involves learners producing language using the target content freely.

Example
The teacher has shown the learners the form and use of the past passive form. They now practise using the structure by completing sentences using cues, e.g.
My car (stolen) _________ last week.

In the classroom
Controlled practice can still be meaningful despite its limitations. For example, we could ask the learners to complete the following based on their experiences:
(What?) was stolen (when?).
This is even more controlled in terms of the target language but allows the learners to personalise the activity.

Further links:

https://www.teachingenglish.org.uk/article/planning-a-grammar-lesson