QWERTY Home row keys: asdf jkl;
Home row keys of a Qwerty keyboard - are keys above which your hands are positioned. On Qwerty keyboard home row keys for the left hand are: ASDF with left hand forefinger positioned on F and right hand home row keys are: JKL; with right hand forefinger being positioned on J.
Hands positioning over Qwerty keyboard - Left hand is position over left hand home row keys with left forefinger lightly touching F key, middle finger D key, ring finger S key and little finger A key. Right hand is position over right home row keys with forefinger touching J key etc.. From home key you reach to a key you want to type and return back to home key. Home row keys are your base from where you reach out and return to. Usually, one of home row keys, one for each hand, has a little dent which lets you know, without looking, that your fingers are positioned above the key board correctly.
Errors - acceptable number of typing errors is one error per minute regardless of how fast you type.
wpm - words per minute typing rate. Word is consider to be 5 key strokes including spaces. If you type 50 key strokes per minute your typing rate is 10 wpm.
Rhythm - you must learn to type with even rhythm. Good even rhythm will rid you of tension, anxiety and will minimize mistakes made. Instead of focusing on "quick where is the next letter" even rhythm will let you focus on what to type. If you slow down to achieve even rhythm you will in effect speed up.
About typing lessons for Qwerty keyboard - Qwerty keyboard was great when people were using typewriters. Today computer keyboard do not need Qwerty layout. Learning to type Qwerty style is an effort to be backward compatible. If you don't know why to learn typing Dvorak or why was Dvorak keyboard layout developed.