The simplest explanation is that the {} brackets group multiple statements into a single statement (that a drastic simplification)
Take if for example
if (condition) statement else statement
that might be if (a == 10) b = 20; else b = 42;
or it might be
if (a == 10)
{
b = 20;
do_something();
}
else
{
b = 42;
do_something_else();
}
{} - Arduino Reference for a better description