The word 'erection' can apply to the construction of any building but in terms of 'sex' it means when the boys penis becomes long , hard and stiff.
Why do boys get erections?
It can occur as a result of direct sexual stimulation
of the penis, but it can also occur when boys are just thinking of sexy things, or looking at photos or other pictures which they find sexually stimulating or reading about something they find a sexual turn on. They can also occur at other times like when you have full bladder and want to pee, or can happen spontaneously
particularly in the mornings on waking up - the so called 'morning stiffy' or 'morning glory'.
The male penis contains, under the skin, two tubular structures which can become engorged with blood - these are called the 'copora carvernosa'. There is a third tube along the penis called the corpus spongiosum above the corpus carvernosa which can also get filled with blood and along which runs the tube - the urethra - through which boys pee.
When boys get erections muscles at the base of their penis make sure that more blood flows into these tubular structures in the penis than flows out so that the penis grow hard and long. The result is the boy's penis stiffens, increases in length and in girth. If the boy has a foreskin this usually gets pulled back, and the skin of the boys ball sack (scrotum) tightens pulling his balls up towards the base of the penis. When the extra blood flows out of these tubes then the boy's penis goes floppy (flaccid) again.