The initial part of the script is used to write the header part of the HTML page.

The following script redirects all the contents up to EOF1 to index.html:

cat <<EOF1 > index.html 
contents... 
EOF1 

The header includes the HTML and CSS styling.

for img in *.jpg *.JPG; iterates over the filenames and evaluates the body of the loop.

convert "$img" -resize "100x" "thumbs/$img" creates images 100px-wide as thumbnails.

The following statement generates the required <img> tag and appends it to index.html:

echo "<a href=\"$img\" >" 
echo "<img src=\"thumbs/$img\" title=\"$img\" /></a>" >> index.html 

Finally, the footer HTML tags are appended with cat as  in the first part of the script.