HAProxy stands for High Availability Proxy, If you are running the Debian Jessie-8 operating system and install  HAProxy 1.5-stable version

You can install the HAProxy using below comamnds on Debian Jessie-8    

# apt-get update

# apt-get install haproxy

You will get HAProxy 1.5-stable version, which means you may not a latest one. 

First, you need to enable the backports repository in  Debian Jessie-8

# echo deb http://httpredir.debian.org/debian jessie-backports main | \
      sed 's/\(.*-backports\) \(.*\)/&@\1-sloppy \2/' | tr @ '\n' | \
      tee /etc/apt/sources.list.d/backports.list

Then, enable a dedicated repository

# curl https://haproxy.debian.net/bernat.debian.org.gpg | \
      apt-key add -

# echo deb http://haproxy.debian.net jessie-backports-1.5 main | \
      tee /etc/apt/sources.list.d/haproxy.list

then, execute the below commands,

# apt-get update

# apt-get install haproxy -t jessie-backports-1.5

You will able to access the  latest release of HAProxy 1.5-stable.