Ttyjfyk

sort -k 2 filename.txt means that it sorts by the second key but what would this following command mean: sort -k 3.3,3.5 for this data:
For example: I got this-->

Man in Winter England 1980.12.02 
Richrd Fritz Scottland 1960.12.18
Max Winter GB 1955.12.09
Luther Arnold England 1990.05.12
Sebastian Kalle USA 1980.12.14

How can I get the solution for that list with this command:
sort -k 3.3,3.5 data.txt?

And how can you sort it by when you have 2 decimal numbers in general?

Let's assume there are no tabs in the input. The interpretation of the command is pretty tricky:

sort -k3.3,3.5

means "sort by a substring from the third field from the third to the fifth character", but the counting begins at the first whitespace before the field, as mentioned in man sort:

KEYDEF is F[.C][OPTS][,F[.C][OPTS]] for start and stop position, where F is a
field number and C a character position in the field; both are origin 1, and the
stop position defaults to the line's end. If neither -t nor -b is in effect,
characters in a field are counted from the beginning of the preceding whitespace.

Run the sort under LC_ALL=C to avoid locale influencing he sort order.

Note how the order changes if you add one more character, i.e.

LC_ALL=C sort -k3.3,3.6

Here's a short Perl script that shows what part of the input is used for sorting:

#!/usr/bin/perl
use warnings;
use strict;
use feature qw say ;

my $field_index = 3;
my $start = 3;
my $stop = 5; # Change to 6 to explain the different order.

while (my $line = <>) 
 chomp $line;
 my @fields = $line =~ /(s*S*)/g;

 my $length_before = 0;
 $length_before += length $fields[$_] for 0 .. $field_index - 2;

 my $from = $start - 1 + $length_before;
 my $to = $stop + $length_before;
 $_ > length $line and $_ = length $line for $from, $to;

 substr $line, $to, 0, '>>';
 substr $line, $from, 0, '<<';

 say $line;

Output for 3.3,3.5:

Luther Arnold << >>England 1990.05.12
Man in << >>Winter England 1980.12.02 
Max Winter << >>GB 1955.12.09
Richrd Fritz << >> Scottland 1960.12.18
Sebastian Kalle << >> USA 1980.12.14

Output for 3.3,3.6:

Richrd Fritz << >>Scottland 1960.12.18
Sebastian Kalle << >>USA 1980.12.14
Luther Arnold << E>>ngland 1990.05.12
Max Winter << G>>B 1955.12.09
Man in << W>>inter England 1980.12.02