Learn Italian: L'arbitro sta guardando, difendete bene! - The referee is looking, defend well!

This is the informal imperative (command) form for voi (you all). For regular -ere verbs like difendere (to defend), the voi command is exactly the same as the present tense: difendete. The speaker is telling a group of players on the field to defend.

Are the voi and noi command forms always the same as their regular present tense forms?

Yes! Unlike the tu form, which has a special ending for -are verbs, the commands for noi (let's...) and voi (you all, do...) are identical to the regular present tense forms. So difendete can mean both 'you all defend' as a simple fact, and the command 'defend!' depending on the context.

What grammar structure is used in sta guardando?

This is the present continuous tense, which you use for actions happening right at this exact moment. It is made by using the present tense of stare (here, sta for he/she/it) plus the gerund form of the main verb, which ends in -ando for -are verbs like guardare.

Why use guardando (looking/watching) instead of vedendo (seeing)?