These are the IBM backend overview, do: Avg.T1/T2: here stand for the Average decoherence time or coherence time?
Both T1 and T2 are quantum decoherence times.
T1 relaxation time is the time taken by qubits to decay from |1> to |0> while
T2 dephasing time is where a qubit dephases from |+> or |-> into a mixture of phases such that the phase cannot be accurately predicted
Quantum decoherence time is the amount of time that the qubits preserve its quantum properties. so, the decoherence time is measured by T1 and T2.
You can imagine coherence as heating up a cup of water, and decoherence is mean how fast the cup of hot water cools down.
For more detail answer:https://quantumcomputing.stackexchange.com/a/9632/20884