Tenere literally means to hold (hold/tieni this box for a second) but in Italian can be used (even though the literal meaning is completely different) as a substitute of the verb "ricevere" (to receive/to get): Tieni/(receive or get from me) il sale. "Ecco" can be translated in "here it is/here's" Here's the salt/ Here it is. So in your example they basically mean the same thing: receive/get this thing you asked me. They have the same meaning in that example here but sometimes one can't be used instead of the other one. "Here he is, that stupid guy!"= Eccolo, quello scemo! You can't say "Tieni quello scemo" because here it would mean "Hold that stupid guy!" and that's absurd. But when you ask for an object and they give it to you they are interchangeable.

Ecco + lo = here it is, here he is Eccoli = here THEY are Eccoti: here you are What about a sentence with only 'ecco'? Consider the /ecco/ form as an abbreviation for 'eccolo', so it is used as a kind reply when giving something you were just asked for. Mi passi lo zucchero per favore ? Ecco. (while giving the sugar to the person) In these cases (at the table) /ecco/ is more frequent than /eccolo/. What about /ecco qua/ ? 1) It is done 2) here it is 3) Look what just happened ! (suppose somebody drops something to the floor, and it breaks)

Thank you Alex and Andrew for responding to my question!