1998 Hyundai Accent

HI SIR,

WHAT IS VALVE SEAL? WHAT DOES IT DO? MY CAR IS SOMETIMES SMOKEY WHEN IT RUNS. IM JUST GOING AROUND ASK ING FOR AN OPINION SO I CAN BUY THE PARTS. WHEN I CHANGE THE OIL AND THE OIL FILTER LAST MONTH, THEN CHECK THE OIL DEEP STICK THE FOLLOWING MONTH IT INDICATES VERY LOW LEVEL OF OIL. I FIND IT VERY IMPOSSIBLE TO BELIEVE IT. WHAT CAN I DO TO SOLVE THIS PROBLEM? HOPEFULLY YOU CAN GIVE SOME ANSWER I WOULD REALLY APPRECIATE IT.