Your words compel the listener to comply, no matter the instruction, however you can face the consequences of these words if caution isn't exercised.

For instance, if the caster commanded a creature to "burn" that creature would ignite should it fail to resist the command, if it succeeded the target would suffer a more tame flame and the caster would burn, the severity being determined by how well the target succeeded.

When you influence a creature it can resist with a contested Charisma check against you; you can expend spell slots to increase your bonus to this check equal to the total level to spell slots expended.

Any creature that can hear you speak and understands you must either comply or resist, unless you choose to exempt it when you issue the command - for each creature you exempt you get a -1 from this commands contested check. You only roll for the check once and all targets resisting roll against that result.

If no creatures can understand you then nothing happens.