A light-year is the distance light travels in a year. Light crosses interstellar space at 186, 000 miles (300, 000 kilometers) per second and 5.88 trillion miles (9.46 trillion kilometers) per year. With current technology, it would take us approximately 37,200 years to travel the distance of one light-year. If you were traveling at a speed close to that of light, the time you spent inside the ship would be much slower than the time outside the ship.
Individual human beings don't live billions of years, so they couldn't travel billions of light years at the speed of light. If the spacecraft were traveling at the speed at which Helios 2 was traveling, it would have traveled one light-year in 4269 light-years. The first discussions about the speed of light began with the Greek philosopher Aristotle, who considered that light traveled instantaneously. If you wanted to travel with a constant acceleration of 1G and then a deceleration to the very edge of the observable Universe.
Except that time dilation means that, for those traveling close to c, time would slow down to almost nothing. What I didn't mention is that this acceleration ruins time for you and for people who don't travel with you. If a Saturn V rocket that took man to the Moon traveled, it would take 108,867 years to travel. Space is squeezed in the direction you're traveling, so, for you, you didn't travel as far as the stationary observers saw you travel.
Let's expand on this and travel to the center of the Milky Way, located about 28,000 light-years away. If a spacecraft traveled at the speed of one light-year, it would travel the distance of one light-year in a human year. If you accelerate at that rate for years, you can travel billions of light years during a human lifetime.