Digital cable signal
A signal transmitted over a coaxial cable, like television, is subject to the physical limits of the medium. The signal will weaken over the length of the cable and, if the cable is too long, will eventually become too weak to be usable. You can use higher quality cable to lessen the loss, remove splits or add a signal booster to improve the strength.
The loss of signal over the length of a cable is known as attenuation. It's due to the propensity of the cable to begin acting like an antenna and interacting with radio waves that then add noise to the signal being transmitted through the cable. Television signal strength is measured in decibels - the same unit to measure the volume of sound - and attenuation is calculated by the loss of decibels over a certain distance at a certain frequency. Increasing signal strength is the process of managing attenuation.
Better Cable and Shorter Runs
Two ways to address the attenuation in a cable run are to use higher quality cable and to reduce the length of the cable run. RG6 coaxial cable has an attenuation of 2.05 db over 100 feet and RG11 coaxial cable has an attenuation value that is just 1.30 db per 100 feet, so over each hundred foot run you would save 0.75 db of signal versus the RG6 cable. You can also find ways to decrease the length of the run so there's less opportunity for attenuation to affect the signal. For example, if your cable is run along the outside of your house you could drill a hole and run it through the crawlspace under the house to give yourself a straight shot.
Don't Split Up
Coaxial cable splitters are a handy way to route a signal to different areas, but the splitting also reduces the signal strength as it is split in half - or more. This isn't an issue for a strong signal, but if your signal is on the border, running a splitter could drop the strength below the threshold necessary. Try to get an unsplit signal for the best results. If you find you do need to split the signal you can run it into your television or cable box first and then use the coaxial line out from there to go to your next device. If that still results in a signal that's too weak you can use a powered signal amplifier.
At a point in the cable run before the signal has become too weak to recognize, you can install a powered signal booster that will bring the strength back up for the rest of the run. You need to plug the signal booster into an outlet and then connect your cable run to it, after that is simply amplifies the signal to combat attenuation over a long run.