Help with Android Nextbook Tablet. Says send package to "adb sideload"

Right now screen shows the Android robot on his back with a door open and the background says no commend and in yellow letters says Recovery system v4.2.41 Now send the package you want to apply to the device with "adb sideload <filename>"...

Am not sure what it wants me to do and am not sure where to go if i have to download a file to fix it. Please help!!!!