Effects can Tribute. (Basically every Ritual Spell in existence does this, for example, as do cards like Ectoplasmer and Summoner of Illusions) But in this case, yes, Assault Mode Activate's Tribute is a cost. Ignoring Ritual Spells, Tributing as a cost is significantly more common than Tributing for an effect, but it's not rare enough to ignore entirely.
An Assault monster is an effect monster that has '/Assault Mode' in it's name.
They are upgraded forms of Synchro monsters that are summoned by cards like this one.
For example, if you have 'Stardust Dragon' on your field, you can use this card to tribute 'Stardust Dragon' and summon 'Stardust Dragon /Assault Mode' from your deck.