здесь тесно — it is crowded here
в вагоне было очень тесно — the carriage was packed, the carriage was very crowded
нам тесно в нашей квартире — we are cramped in our flat, our flat is too small for us
тесно так сидеть — we are sitting all squashed together, there is no room for us all to sit here
мне тесно под мышками — it feels tight under the arms