USC Trojans vs. St. Mary's Gaels football game in Los Angeles

The video covers a football game sequence where Stevens kicks off, and the ball advances to Saint Mary's 5 yard line. The play continues with a snap and a tackle, leading to the ball going out of bounds at the 40 yard line. A pass from Saturn results in a gain to the Marion 16 yard line. A lateral play secures a first down for Southern California. The sequence concludes with Wilson executing a long pass on the right side.

Explain the significance of the play described as a lateral.

What does it mean when it says 'first down in one yard to go'?