Decibel is ridiculously overloaded concept.

It can be used to express and calculate relative change in power, amplitude ratios, and absolute change. All of these are different units and should always use different notation, but sometimes it's skipped.

But a lot of them can be combined - like you can take dBm of a microwave signal going into a filter, subtract the dB figure of loss through a filter, add the dB figure of gain through an amplifier, then add the dBi of antenna gain, and get a meaningful number (equivalent isotopic radiated power or EIRP, in dBm) out the end! It’s quite amazing!