Tree spread is simply how wide the trees branches eventually become (from one side of the tree strait thru the trunk to the other side).
Acer palmatum 'Bloodgood' slowly grows to 20 feet wide so it will have 10 foot long branches on each side of the trunk (10+10=20 feet). So measure 10 feet in all directions (360°) from where you want it planted it to make sure it does not contact anything.

Treeguy123, thank you so much for a such a clear and understandable answer. That's EXACTLY the information I was looking for and couldn't for the life of me find it. I really appreciate it!

Cathy in SWPA

Disagree with Treeguy - it usually means the radius of the crown, i.e., the length of the branches spreading out from the trunk. Multiply by 2 to get the total width of the crown.

Resin

In might mean radius in UK or all of Europe for some odd reason, but in the U.S. Spread means just that, the spread of a tree (the diameter measurement of a trees crown). I believe most or all nursers in the U.S. mean this also.
20 feet is the mature width (spread) of Acer palmatum 'Bloodgood', not 40 feet.

Go to this link to see how the average tree spread is calculated for Champion trees:
LINK
For champion trees the Average Crown Spread is made by adding the smallest diameter of the crown and the largest diameter of the crown, then dividing it by two. i.e. 30 ft spread + 20 ft spread = 50 ÷ 2 = 25 foot spread

Here is another example of a tree guide showing tree spread:
25'-35' Spread. Note the spread image below the Loblolly Pine, Diameter not radius.
Link

Some more links:
Link
Link