要紧 (yào jǐn) means 'important, urgent' and is used in everyday speech, often in negative constructions like 不要紧 'never mind, it doesn't matter'. It is less formal and less intense than 紧急 (jǐn jí) or 紧迫 (jǐn pò), which describe a critical, time-sensitive crisis. 要紧 can also simply mean 'serious' in a casual sense, as in 'it's not a big deal'.