The short answer: it sounds bad, doesn’t it? Time to get in touch with me! The long answer:
Pianos are surprising complex: made of various types of wood, felt (hammer heads, damper pads, etc.), leather (buckskin), and metal (cast iron frame, highly tempered steel piano wire, copper, brass, and nickel-copper alloys (center pins). The 88 notes each have one to three “strings” (steel wire) at very specific tensions with which to produce harmonious music. The tensions of these wires change with changes in relative humidity and temperature. The reaction to humidity change is because all the strings are in dynamic tension with the wood bridges (maple) and soundboard (spruce); the bridges and soundboard expand and contract as their moisture content rises (summer) and lowers (winter).
As such, piano manufacturers recommend pianos be tuned every six months.
If it has been longer since your last tuning I will probably have “more work” to achieve an optimal tuning. Because piano wire is under high tension (up to 150 in/lbs each!) the wire gradually stretches, its tension lessens, and the pitch produced goes flat. Therefore a “first pass” of adding tension to each string is necessary, followed by a more exact adjustment of the tension to produce the correct pitch and clarity of tone.