is it obligatory to understand & ponder over the meanings of what we recite in the salah like al fatiha, other surahs, dhikr, attahiyat etc? is this what is meant by having khushu? is it valid if we just focus on reciting in the salah without focusing on the meanings and try to avoid other thoughts?

No, full khushu is not a condition, so your salah is valid if you do its pillars with stillness, even if thoughts come to you, but you only get the reward of what you were present for. Khushu is the presence of the heart before Allah, and the scholars have counted pondering what you recite as part of it, so strive for it.