explain: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/explain - from package: explain

You may install the required package with command yum install {package}

Error in other languages:
explain: bevel nie gevind nie
explain: comandă negăsită
explain: comando não encontrado
explain: commande introuvable
explain: command not found
explain: käsku ei ole
explain: không tìm thấy lệnh
explain: komanda nerasta
explain: Kommando nicht gefunden.
explain: kommandot finns inte
explain: komut yok
explain: nie znaleziono polecenia
explain: níor aimsíodh an t-ordú
explain: no se encontró la orden
explain: no s'ha trobat l'ordre
explain: opdracht niet gevonden
explain: parancs nem található
explain: perintah tidak ditemukan
explain: príkaz nenájdený
explain: příkaz nenalezen
explain: команда не найдена
explain: командата не е открита
explain: 命令找不到
explain: コマンドが見つかりません