toe the line

Short definition from Wiktionary

Verb

1. 1
   To abide by the rules or conventions; (politics, in particular) to follow the party line.
2. 2
   To stand at one's mark before a footrace.
3. 3
   To be on a boundary or in a grey area between two states.
4. 4
   To push boundaries imposed by an authority; to push the envelope; to flirt with crossing the line.