For example... if there's an event in a character's past that has some bearing on his/her current actions... a theatrical movie might have the budget to show the event in a period setting or a different location. A TV series would be able to tease at the possibilities of what the event might be from episode to episode before actually explaining it. A TV movie generally lacks budget and the runtime is fixed, as it's often a cheap daytime schedule filler, so it might just have the characters cover the past event in a few clunky lines of dialogue before getting on with the next scene. Made-for-TV movies also have a reputation for being melodramatic.
Note that made-for-TV as a derogatory term is more a US thing, so I haven't actually
seen many (if any) such movies over here in the UK. Our one-off feature length TV dramas tend to be very high quality
so it's strange to hear that term over here.