I keep hearing that if you hit your head and no bruise, lump or tender spot is there, it indicates a concussion. Is this true?

Likely, bt not alwys. Concussion or mild traumatic brain injury can involve both an indirect and direct hit to the head even if there are no obvious bruises, lumps or cuts. Also, you don't have to lose consciousness to be diagnosed with a concussion. Having your body slammed into the ground (wrestlers or football players) even without any direct hit to your head can cause one.
No. Although no external bruising is necessarily present with concussion, you do not have a concussion unless you have signs of one, including headache, nausea, unsteadiness, confusion altered alertness.