Viscosity is thickness. A lower number is thin, a higher number is thick.
Thicker oil has more resistence to movement, and also more resistance to breaking down under pressure.
In a "multi-vis" oil, the first number is "cold" the second "hot" Remember, oil gets a lot thicker when it is cold, and a lot thinner when it is warm.
So, a 10w oil is the thinkness of 10 wieght oil when it is cold (it stays thinner when cold)
A w30 oil gets pretty thin when hot, w40 not quite as thin, and w50 (most racing oils) stay pretty thick when hot.
Manufacturers have recommended thinner oils in recent years to minimize rotating resistence so they get better fuel economy.
Many, including me, prefer a thicker oil when hot.
Still, a good 30 wieght oil is good for most any street / highway conditions.