The heat-affected zone is the band of parent metal next to a weld bead where the temperature got high enough to change the microstructure (and therefore the strength) but not high enough to melt and become weld metal.
In structural steel, the HAZ can be either harder or weaker than the surrounding metal depending on the steel grade and the welding process. For mild steel C350 RHS, the HAZ is typically slightly weaker than the parent metal because of grain coarsening from the heat. The weld itself, made with overmatched filler (ER70S-6 puts down weld metal stronger than C350 base), is usually the strongest part of the joint.
Designing around HAZ effects: don't put bolt holes in or right next to the HAZ. Don't place fatigue-critical sections in the HAZ. Specify pre-heat for thicker sections to slow the cool-down rate and reduce hardness in the HAZ.