Hola Trefor

Yes, that is the present subjunctive: podamos

It's used here because the speaker is expressing something that is known, so we assume that by saying "que no podamos ir" is something that is already known by the speakers, something that has been talked about previously. 

If what the speaker wants to "declare" that bit of information, i.e. express that as something "new" then they may have said "Entiende que no podemos ir." - however this use of the indicative would be uncommon as it's using the verb "entender que..." and this would mean that the person already knows that information. 

The verb entender here could also be interpreted as a verb of "feeling" as in "being OK with something", which would also trigger that subjunctive: "Entiende que no podamos ir" (~ Acepta que no podamos ir, le parece bien que no podamos ir...) 

Saludos cordiales

Inma