parasitics

At low frequencies a wire is just a wire and a solder joint is just a connection. But every real piece of metal also has a little inductance, every gap between conductors has a little capacitance, and every imperfect connection has a little resistance, whether you drew them on the schematic or not. These uninvited components are called parasitics. Think of them as freeloaders that ride along with your real parts. At DC and audio they are too tiny to notice; at radio frequencies they take over the circuit.

Here is why they wake up. The impedance of an inductor is 2 times pi times f times L, and a capacitor's reactance is 1/(2 times pi times f times C), so as frequency f climbs, stray inductance matters more and stray capacitance shorts things out. A short straight wire has very roughly 1 nanohenry (nH) of inductance per millimetre. At 1 kHz that 1 nH is 0.0000063 ohm, utterly negligible. At 1 GHz the same 1 nH is 2 times pi times 10^9 times 10^-9 = about 6.3 ohms, suddenly comparable to the things you care about. A 5 mm capacitor lead can resonate with the capacitor itself and turn your bypass cap into an inductor above a certain frequency.

Parasitics are why an RF circuit lives or dies by its physical layout, not just its schematic. The schematic can be perfect and the board still fails because the loop area is too big, the ground is shared, or a component lead is too long. Honest framing: parasitics are not a flaw you can eliminate, only manage. Good RF design keeps connections short and direct, uses ground planes, and chooses components whose parasitics are small and predictable, so the strays become part of the design rather than a surprise.