The message indicates that the charger is not giving as much energy as is required for your computer not to lose charge when working normally. If you use the computer lightly, or you suspend it, or shut it down, you will get a slow recharge.
The cause for the low charge can be either a dirty Magsafe/USB-C socket, a faulty power supply, a misconfigured System Management Controller (SMC) —as indicated elsewhere—, or a self-resetting fuse in the power supply.
In this last case, removing the power supply from the wall socket and waiting a couple of minutes can resolve it.
If the issue is the SMC, you can follow the method indicated by Apple in their support document about "How to reset the System Management Controller (SMC) on your Mac".
And if the issue is a dirty socket, you can try to use a compressed air can, or something like that, to blow away the dirt.